1. Field of the Invention
The present invention relates to the field of high rotation speed vacuum pumps, which are fastened to and connected to a structure such as a vacuum enclosure or a pipe to generate a hard vacuum. It concerns more particularly a device for fixing such vacuum pumps.
2. Description of the Prior Art
In the electronic and micromechanical component industries, machining or plasma processing methods are executed inside an enclosure in which a controlled vacuum must be maintained.
Generating the vacuum necessitates the use of pumps capable of quickly generating and maintaining a hard vacuum adapted to the machining or processing method. Turbomolecular type pumps are generally used, consisting of a pump body in which a rotor is driven in rotation at high speed, for example at more than thirty thousand revolutions per minute. With such high rotation speeds, the rotor acquires a very high kinetic energy.
The pump bodies include a coaxial inlet orifice that is connected to an outlet orifice of the structure such as the vacuum enclosure or the pipe. The pump is generally fastened only to the structure, and is supported only by the area surrounding the pump inlet orifice and the corresponding orifice of the structure. The pump body therefore includes a coaxial annular flange around the inlet orifice, threaded holes are provided on the wall of the structure around the outlet orifice, holes are provided through the coaxial annular flange of the pump body, and headed bolts are fitted so that their shanks pass through the latter holes and bolt into threaded holes in the wall of the structure to fasten the vacuum pump to the structure with the flange pressed against the wall of the structure.
The shanks of the bolts are conventionally cylinders of circular section, with a smooth portion that passes through a hole having a diameter slightly greater than the diameter of the shank, and with a threaded end portion that is screwed into an associated threaded hole.
Standards specify the respective dimensions of the flange, the bolts and the necessary holes, together with the number of bolts and holes, as a function of the pump diameters.
Accordingly, for an ATHM type turbomolecular pump from ALCATEL, the pump is fixed by providing a flange of the DN 250 iso-F type conforming to the “PNEUROP 66061” standard, with twelve bolts of M10 type the shank whereof has a length of approximately thirty millimeters for a diameter of ten millimeters, and the holes in the flange and the structure have a nominal diameter of eleven millimeters.
Such fixing structures are entirely satisfactory under normal conditions of use, and are thus able to withstand the mechanical forces generated by the operation of the vacuum pump in normal use.
On the other hand, it has been found that an insurmountable problem may arise in the event of accidental destruction of the pump rotor when rotating at full speed. On this assumption, the rotor running at the full rotation speed is then out-of-balance, may violently strike the wall of the pump body, imparting to it a transverse or radial displacement force, and may rub strongly on the wall of the pump body, imparting to it a coaxial rotation torque. Because of the high energy stored in the rotor rotating at high speed, the mechanical stresses applied by the rotor to the pump body are very high, and those stresses are transmitted to the device fixing the vacuum pump to the structure. This results in violent shear forces on the fixing bolts, and it has been found that in current structures these forces cause the bolts to break. The pump is then detached from the structure and constitutes a dangerous projectile that can fly around the place of use.
One solution is proposed by the document WO-2004/020 839, which describes a device for fixing a vacuum pump to a wall of the structure, comprising an annular flange coaxial with and fastened to the vacuum pump body around the inlet orifice. Threaded holes are provided on the wall of the structure, through-holes are provided in the annular flange, and headed bolts are fitted so that their shanks pass through the through-holes and are screwed into the associated threaded holes to fasten the vacuum pump to the structure, with the flange pressed against the wall of the structure. The through-holes comprise a distal portion in the shape of a circular section cylinder followed by a coaxial enlarged proximal portion of circular section adjacent the wall of the structure. In the event of shear forces in any lateral direction in the area of the connection between the vacuum pump and the structure, this conformation allows flexing of the bolt shank and a corresponding lateral offset between the through-hole and the associated threaded hole.
This solution absorbs the violent shear forces to which the fixing bolts are subjected. However, the rotor rotation speed is higher in new generation turbomolecular pumps. In the event of a pump crash, the energy that must be dissipated and not transmitted to the structure becomes much greater and in this configuration the yield strength prior to the bolts breaking becomes insufficient. Furthermore, this solution does not prevent the pump from becoming detached from the structure in the event of the bolts breaking.
An object of the present invention is to eliminate the drawbacks of the prior art. To this end the invention proposes to modify the vacuum pump fixing structure to increase the strength with which the vacuum pump is retained on the structure and to prevent the fixing bolts breaking and the pump separating in the event of the rotor bursting while running at full rotation speed, whilst remaining compatible with the applicable standards.